The invention relates generally to rotary positioning systems and more particularly to a system and method to provide absolute referencing in rotary positioning systems.
Typically, a rotary positioning unit includes a motor (e.g. electric, hydraulic etc.) and a speed reducer whose input is driven by the motor and whose output delivers a rotary motion to a load. Here, an angular position of the output drive shaft is measured using a rotary encoder placed on the motor shaft or at the output of the speed reducer, or both. In particular, the angular position measurements are obtained using a relatively low-cost incremental encoder or a high-cost multi-turn absolute encoder on the motor shaft, or a very high-cost high resolution absolute encoder at the output shaft.
However, the incremental encoders have to be referenced every time on power-up. This process of providing a repeatable reference is generally referred to as “Mastering”. Also, mastering requires the output drive shaft of the rotary drive to be programmatically rotated to an electronic limit switch at one end of its motion range and the sensing of the limit provides a reference signal for initializing the encoder count to a user-programmable value. This process becomes problematic when there are multiple rotary positioning units in a system and even more so when they are inter-dependent (For example, as in a robotic arm). In some cases, mastering becomes tedious when the output drive shaft referencing limit switch is outside the range of operation.
Currently, these drawbacks are addressed using expensive multi-turn but low-resolution absolute encoders on the motor shaft or even more expensive single-turn high resolution absolute encoders at the output drive shaft of the speed reducer, or by providing a battery backed-up non-volatile storage of the angular position measurement from the incremental encoder. However, the resolution of absolute encoders when mounted at the joint output drive shaft has to be relatively high for precision motion applications (typically of the order of 24 bits or more for one single turn of the output drive shaft). Also, single turn absolute encoders are relatively inexpensive for a small number of bits (8 or so) but climb rapidly in cost and size as they provide from 16 to 30 bits.
Moreover, battery backed up non-volatile storage solutions require monitoring of battery life and periodic battery replacement and cannot address scenarios in which the rotary positioning unit is disturbed manually or otherwise during the power loss period. In such a scenario the stored angular position measurement does not account for the disturbance.
Therefore, there is a need for a cost effective system for mastering of the rotary positioning unit.